This invention relates to structures and methods for producing a linear array of streaming plasma with a very low level of contamination and low energy ions in a vacuum processing chamber, e.g., such as in those used to process compound films (e.g., oxide films), to synthesize thin films on the surface of a substrate.
In a vacuum processing chamber ion sources are used to change the properties of substrate surfaces. Gas is fed through an electric field in a vacuum chamber to excite the gas to a plasma state. The energized ions or excited neutrals (such as excited atoms and disassociated molecules) of gas constituents bombard the surface of the substrate. The effect that the ions have on the surface is dependent on their atomic constituents and their energy (in one example to treat a surface by providing active oxygen).
Low-energy, ultra-clean flows of plasmas are required, to obtain a crystalline film with high crystal quality as required in the case of gallium nitride (GaN). To avoid ion damage the ion energy must not be too high, for instance, an ion with a kinetic energy of 20 or more eV for GaN must be considered a high energy ion. When an ion hits the surface during crystalline film growth and its kinetic energy is high enough to displace atoms which are already in place, a defect is created. So the materials grown with energetic plasma streams tend to have lots of defects. The defects create xe2x80x9ccarrierxe2x80x9d densities for semiconductors such as GaN, leading to the creation of a material with n-type doped properties. In these cases it is difficult to obtain a p-type doped material. To overcome this problem a source of low energy ions is needed.
A nitrogen plasma flow or low-energy ion beam (ion energy of order 30-50 eV) is usually obtained by using a Kaufman ion source or an Electron-Cyclotron-Resonance (ECR) plasma source.
The drawback of a Kaufman source is that a hot tungsten filament is used as a cathode. The tungsten filament delivers large quantities of electrons by thermionic emission, which sustain the low-energy non-self-sustained arc discharge, but tungsten atoms evaporate from the filament and can be found in the stream of plasma and the growing films. This is not acceptable for instance in the growth of GaN because the stream is not clean and the film properties are altered by the impurities, and is not acceptable for growth of oxide films, because the tungsten filament will oxidize rapidly.
An ECR plasma source necessarily operates with a high magnetic field to fulfill the resonance condition of microwave frequency and electron cyclotron frequency. Typically, the standard microwave frequency of 2.45 GHz is used, leading to a required magnetic field of 875 Gauss. The gaseous microwave plasma is produced in the region of the resonance magnetic field. The ions gain kinetic energy when leaving the location of the high magnetic field and streaming towards the substrate. When a plasma is made this way there is a significant energetic component in the ion energy distribution, i.e., ions having 30-50 eV of kinetic energy are abundant. This energy is too high for the growth of a high quality crystalline films. Although ECR plasma sources are cleaner than Kaufman sources, ion damage is observed in growing films due to the relatively high ion energy. One way to overcome this energy problem is to bias the substrate electrically, to deflect the energetic ions but in doing so the low energy ions are also deflected and the growth rate decreases.
A better (cleaner) source of low-energy gaseous ions is needed to deposit high quality thin films on substrates in both research and commercial applications. This need includes not only MBE-type but also IBAD-type deposition of thin films (MBE=molecular beam epitaxy; that is film growth with reactive, activated gases; IBAD=ion beam assisted deposition, that is film growth assisted by the moderate kinetic energy of ions such as argon).
A plasma discharge chamber usable for an ion beam source, electron beam source, and a spectral light source was introduced by V. I. Miljevic and is described in several papers (Rev. Sci. Instrum. 55 (1984) 931; Rev. Sci. Instrum 63 (1992) 2619) (also see U.S. Pat. Nos. 4,871,918; 4,906,890). A preliminary explanation of the working principle of the discharge is given in a paper published in Plasma Sources Science and Technology, Vol. 4. (1995) p.571.
In one configuration as shown in FIG. 1, a gas flows through a discharge chamber 20 which consists of a metal cathode 22 (grounded) and a metal anode 24 (positively biased) separated by a Teflon insulator 26. A flange 28 holds the anode 24 in place and seals it against the cathode flange using a series of O-rings 30. By applying a sufficiently high voltage (500 V or more) to the electrodes, a glow discharge ignites in the flowing gas. The gas is introduced through an opening 40 in the cathode, and leaves the source through a small aperture 38 in the anode. A high positive voltage is applied to an extraction electrode 32 leading to acceleration of the ions from the source 20 in the direction shown by arrow 36. A high negative voltage would accelerate electrons, turning the source into an electron beam source. An electromagnetic coil 34 produces a magnetic field around the anode 24 focusing the ions in an ion beam (whose direction is shown by the arrow 36) departing from the discharge opening 38 in the anode 24. Gas pressure supplied to the gas inlet 40 provides the motive force to discharge the ions from the discharge chamber 20. The extraction electrode 32 and magnetic coil 34 assist in accelerating and focusing the ion discharge into a beam. The anode 24 is insulated from the grounded cathode 22 and grounded support flange 28 by a thin film of ceramic coating deposited on the respective mating surfaces of the anode 24.
The feature which distinguishes this kind of discharge from an ordinary glow discharge is the actual exposure of a very small area of a large cross section anode facing the cathode, to the gas. In the Miljevic configuration this effect is obtained by blocking nearly all of the anode 24 by using an insulator 26, except for a small discharge aperture. This discharge aperture forms a small hollow anode, and Miljevic named the discharge xe2x80x9chollow-anode dischargexe2x80x9d. Related research has found (Plasma Sources Science and Technology, Vol. 4. (1995) p. 571.) that a voltage drop appears in front of the discharge opening, accelerating electrons which gain enough energy to ionize the working gas through inelastic collisions. A bright xe2x80x9canode plasmaxe2x80x9d forms in the anode channel, and this plasma is blown out by the gas flow in the channel due to the pressure gradient between the inside and the outside of the source. The xe2x80x9canode plasmaxe2x80x9d does not form when there is no blocking or covering such a large anode.
A structure and method according to the invention involves using a special type of glow discharge plasma source, namely the so-called xe2x80x9cConstricted Glow Discharge Plasma Sourcexe2x80x9d. The configuration of the prior art plasma source is adapted in a way that it delivers a downstream gaseous plasma of low contamination and very low kinetic energy, well-suited for the growth of high-quality thin films. The source can operate in a wide range of parameters, in particular it can also work at very low and very high gas pressures. It has been found that the anode does not necessarily need to form a small opening which is located next to the blocking insulator. The xe2x80x9chollow anode dischargexe2x80x9d is just one possible configuration which makes use of a constriction element. A configuration according to the invention includes a special type of glow discharge characterized by a constriction between cathode and anode. The inventor(s) named this type xe2x80x9cconstricted glow discharge,xe2x80x9d and the derived downstream plasma source xe2x80x9cconstricted glow discharge plasma source.xe2x80x9d
The constricted glow discharge plasma source includes a discharge chamber where the potential of the cathode is below that of the anode, and the potential of the anode is approximately the same as the potential of the substrate being processed, thereby minimizing the energy of ions reaching the substrate. Having the substrate and anode at substantially the same potential, eliminates the accelerative effect on ions or electrons that a biased substrate would experience. Ion energy can be adjusted by biasing the substrate or changing the plasma potential (changing the anode potential). The configuration and method according to the invention provides a simple construction and operation of a low energy plasma source.
The source is simple, compact, and versatile. It can be used with a large number of surface modification techniques and thin film synthesis methods. Synthesis implies that some sort of chemical reaction occurs while depositing plasma constituents. A few applications which are already known include:
Growth of high-quality GaN thin films. GaN is a wide bandgap semiconductor with a number of applications such as blue light-emitting diodes, flat panel displays, and high temperature electronics applications. The constricted glow discharge plasma source has been shown to be a key element in achieving the required film quality by an MBE-type growth.
Gas streams in all versions of the source are fed to the discharge chamber. The electric field there causes the gas to become partially ionized and leaves the source through an constriction element located upstream of the anode.
The streaming plasma contains only low-energy ions (lower than 20 eV) because (1) the anode is positive, therefore attracting electrons but decelerating ions, and (2) a relatively dense, collisional plasma is formed in the upstream vicinity of the constriction element, in which energetic ions and neutral atoms lose their energy by collisions. The requirements of low ion energy (a few eV or less) for GaN film growth is therefore fulfilled.
The plasma stream from the source is clean because (1) no filament or other hot parts are used, (2) material sputtered by the ion bombardment from the cathode is - with a very high probability - deposited inside the discharge chamber since the output aperture of the constriction element is very small, (3) the source can be built of material tolerable or desirable to the specific application. For instance, a source has been construed where all plasma-facing components are made of high-purity aluminum (electrodes) and high-purity aluminum nitride ceramics (insulator parts). Such a source makes growth of high-quality thin films, such as GaN, possible.
It has been found that cathodes made from titanium show excellent long term stability when operating with nitrogen. A TiN film is formed on the surface of the cathode as a result of a chemical reaction of the cathode with the activated plasma nitrogen. Titanium nitride is sufficiently conductive to sustain the cathode function, but does not form an electrically insulating film as AIN eventually does.
Similarly, stainless steel (e.g., SS 304L) has also been used successfully as a cathode with oxygen because it does not form an insulating oxide film.
In general, a criterion for the selection of a cathode material for a given gas is that this material, if it reacts chemically with the gas, does not form an insulating film on the cathode surface. However, it may well form a conducting film.
An optimization of the configuration according to the invention, in particular the cathode shape and material adds new features to the source:
operation at an even wider range of pressures,
operating with reactive gases such as oxygen,
operating with unusual gases such as water vapor (for example it might be desirable to include a pre-defined amount of water or its constituents in films),
increased plasma output and stability, and
higher power and improved cooling.
A configuration according to the invention includes a source of plasma having a hollow enclosure with a gas inlet opening and a gas discharge opening. A cathode surface is exposed to an interior space of the enclosure and an anode surface is located downstream of a constriction element, a gas is fed to the gas inlet opening and plasma is emitted from the discharge opening of the constriction element. The shape of the opening of constriction element can be round or elongated (rectangular). In other configurations the source is fluid cooled (liquid or gas) and/or the cathode is insulated from the surrounding environment, so that all exterior surfaces of the source are safely at ground potential. The cathode can be a hollow configuration to increase its surface area, and the inlet gas passage can include a small inlet orifice to prevent a discharge in the gas feed line. The geometric relationship between the substrate and the source may be fixed or variable during processing.
In another configuration according to the invention, two plasma chambers are constructed in series; the first chamber feeds the second chamber with plasma. This increases the stability and density of the plasma emitted and reduces the probability that a high energy particle will be generated and leave the second chamber through its discharge opening of the constriction element.
The plasma source cells can also be configured in parallel, to provide several plasma streams toward a substrate.
The source can be constructed and operated in a way that the anode is remote from the small discharge opening of the constriction element. This can be done, for instance, by applying the negative potential to the cathode as previously described but keeping the other source parts (such as housing and the other metal parts adjacent to the blocking insulator) electrically floating. The anode can be a separate ring or tube located downstream of the main body of the plasma source. The anode can be attached to the main body (forming a unit) or detached from it. In an extreme case, the substrate, substrate holder, and the chamber wall can function as the anode, and no dedicated anode part is necessary. In all cases, the discharge is constricted to a small area (xe2x80x9cdischarge opening of the constriction element.xe2x80x9d) such that a voltage drop forms at this flow constriction. The flow of current between the interior cathode and the exterior anode is concentrated at the flow constriction. This causes plasma production as described above, and the plasma formed at the flow restriction is blown to the substrate by the pressure gradient.
The source can operate with all kinds of gases. Besides nitrogen, sources have been tested running with argon, air, water vapor, ammonia and oxygen. The latter is important, for instance, for the deposition of oxide films. Since the source can operate at a high pressure typical for sputter deposition of thin films, plasma-assisted sputter deposition becomes feasible by combining one or several constricted glow discharge plasma sources with magnetron sputtering and laser ablation facilities. This could have great impact on the deposition of oxide films for controlling the passage of sunlight and electrochromic windows and deposition of high temperature superconducting films such as yttrium barium copper oxide. The use of the source configuration according to the invention with oxygen promotes the enhanced incorporation of oxygen in the surface layer, as the concentration of oxygen is usually too low in the original as-deposited films.
A source according to the invention is well suited to produce a nitrogen plasma used for the MBE growth of gallium nitride films on heated substrates such as sapphire and silicon carbide.
Other gasses excited to a plasma state can be used with a source according to the invention.
Operation of the source with an inert gas such as argon at low pressures can be useful for IBAD thin film deposition.
The lifetime of constricted glow discharge plasma sources in a configuration according to the invention is much longer than sources operating with hot filaments. This is generally true but in particular when operating with oxygen, because hot filaments burn easily in oxidizing environments.
A method according to the invention includes the steps of: feeding a gas into a hollow discharge cell, applying a DC voltage to form a plasma in the discharge cell including a small constriction area such as to provide a downstream plasma flow with low energy ions to help synthesize nitride films, such as gallium nitride, on suitable substrates such as AIN, sapphire, and SiC, and to synthesize oxide films such as tungsten oxide films on glass or other substrates, and to synthesize a yttrium barium copper oxide film by exposing such films to an oxygen plasma to obtain an increased concentration of oxygen atoms in the film, and to assist the growth of thin films such as metal films by providing low energy ions such as argon ions.
A device according to the invention provides for
(1) Plasma assisted deposition of oxide films on large area substrates, including dielectric substrates such as glass and plastics (webs).
In one case, the source runs with oxygen as the feed gas. The oxide films can be: (a) indium tin oxide, a transparent, conductive coating (part of multilayer electrochromic films (variable optical transmission) or heatable glass for luxury car windows); (b) ion conducting oxide films such as tungsten oxide (part of the electrochromic multilayer structure); (c) solar control films (such as zinc oxide films); and (d) anti-diffusion barrier films such as aluminum oxide (used, for instance, on plastic packaging for potato chips; the coatings prevent the diffusion of water and oxygen thus help to keep the food fresh).
(2) Plasma assisted deposition of nitride films.
The source can operate with all gases, and when used with nitrogen, nitride films can be deposited.
A most interesting and promising version of the Constricted Plasma Source (CPS) is a linear array of miniaturized discharge cells. The arrangement as with sources in parallel was only recently reduced to practice because each source in the earlier configuration was too large in diameter to allow the necessary close spacing needed for a homogeneous plasma. Close spacing of the sources (discharge cells) is mandatory to obtain acceptable plasma homogeneity along the array.
A device according to the invention may include a plasma source in which the constriction is not circular but narrow slit (or slot). This allows the formation of a xe2x80x9clinearxe2x80x9d plasma. However, when the slit becomes very long (say, several inches), the plasma tends to concentrate at some part of the silt thus the output is not homogeneous along the whole slit length. Combining the pencil-sized source with the quasi-linear array as mentioned earlier, a quasi-linear source with acceptable homogeneity could be reduced to practice for the first time.
The term xe2x80x9cquasi-linear CPSxe2x80x9d refers to a multicell Constricted Plasma Source whose discharge cells are aligned and operate with constricted glow discharge cells with small circular constrictions. The term xe2x80x9clinear CPSxe2x80x9d refers to a multicell Constricted Plasma Source whose discharge cells are aligned and operate with constricted glow discharge cells having slit-shaped constrictions.
A device according to the invention includes an elegant and compact small diameter source design. This miniaturized source can be used in two ways. First, it can be used as a stand-alone device for instance in the geometry required for ultra-high-vacuum molecular-beam-epitaxy deposition facilities. Second, the small diameter of a plasma source cell allows several of them to be arranged closely spaced in a linear fashion, forming the quasi-linear CPS, as discussed earlier.
The miniaturized discharge cell can also have a slit rather than a circular hole as the constriction. The slit would be relatively short (some mm) in this case because the overall discharge cell is small. Many of these smaller slit sources can be arranged to form a linear plasma source of arbitrary length.
The cells of a quasi-linear or linear CPS can be tuned to improve homogeneity of the plasma process such as the deposition of a compound film. Moreover, a desired or programmed output profile can be obtained. A flat, homogenous and constant output profile is one example of a possible desired output.
(1) The cells of the CPS can be operated independently from each other by using
(a) independently controlled power which may, for instance, be obtained by operating individual power supply modules in current limit mode
(b) independently controlled individual gas feeds Methods 1(a) and 1(b) can be used together or independently to achieve a desired output profile.
(2) The distance between individual source cells can be adjusted and optimized for a desired profile. For instance, if a homogeneous plasma profile is desired, the cells in the center of a quasi-linear source are nearly spaced equally but the distance between cells decreases the closer a cell is at one of the ends of the arrangement.